San Antonio Mayor Julian Castro — like the first lady and the president, a graduate of Harvard Law — will deliver the keynote speech that evening, organizers said. Castro will be the first Latino tapped for a keynote speech at a Democratic National Convention, they said.

The convention, held in Charlotte, N.C., runs through Sept. 6.

"Having both the first lady and Mayor Castro speak on the opening night of our convention will bring together two incredible leaders whose life stories both embody the promise of America, that if you work hard and play by the rules, you can make it," said Los Angeles Mayor Antonio Villaraigosa, the convention chairman.

He said the first lady will "offer unique insights into the president as a husband, father and a leader over the last four years."

The youngest mayor of a major U.S. city, Castro was raised by a single mother and attended public schools before he and his identical twin, Joaquin, received bachelor's degrees from Stanford University and law degrees from Harvard, organizers said.

"As mayor, Julian Castro has worked tirelessly to move San Antonio forward by building its economy from the middle out, not the top down," Villaraigosa said.

Earlier, organizers announced that former President Bill Clinton will address the convention Sept. 5 and that Vice President Joe Biden will introduce President Obama before the president delivers his acceptance speech Sept. 6 from the Bank of America Stadium.